where \(h_{+,\times }\)  is the GW amplitude for the two polarizations, the prime denotes the derivative with respect to conformal time \(\eta \) , \({\cal H}=a^{\prime }/a\)  and \(a(\eta )\)  is the FRW scale factor.
In modified gravity, even in theories that do not change the coefficient of the \(c^2k^2\)  term (that would result in a different speed of GWs), still the
“friction term" proportional to \(\tilde{h}^{\prime }_A\)  is different, and eq. (REF ) becomes [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]},
\(\tilde{h}^{\prime \prime }_A +2 {\cal H}[1-\delta (\eta )] \tilde{h}^{\prime }_A+c^2k^2\tilde{h}_A=0\, ,\) 
